Method for controlling gallium content in gadolinium-gallium garnet scintillators

ABSTRACT

Disclosed herein is a method including manufacturing a powder having a composition of formula (1),
 
M 1   a M 2   b M 3   c M 4   d O 12   (1)
         where O represents oxygen, M 1 , M 2 , M 3 , and M 4  represents a first, second, third, and fourth metal that are different from each other, where the sum of a+b+c+d is about 8, where “a” has a value of about 2 to about 3.5, “b” has a value of 0 to about 5, “c” has a value of 0 to about 5 “d” has a value of 0 to about 1, where “b” and “c”, “b” and “d”, or “c” and “d” cannot both be equal to zero simultaneously, where M 1  is a rare earth element comprising gadolinium, yttrium, lutetium, scandium, or a combination of thereof, M 2  is aluminum or boron, M 3  is gallium, and M 4  is a codopant; and heating the powder to a temperature of 500 to 1700° C. in an oxygen containing atmosphere to manufacture a crystalline scintillator.

BACKGROUND

This disclosure relates to a method for controlling gallium content ingarnet scintillators during a manufacturing process. In particular, thisdisclosure relates to a method for controlling gallium content ingadolinum-aluminum-garnet scintillators during the manufacturingprocess.

Gadolinium aluminum gallium garnets (commonly known as GAGG) arepromising candidates for use as a scintillator in time of flight (TOF)positron emission tomography (PET) because of its high density of 6.63grams per cubic centimeter (g/cc), high light output of greater than65,000 photons/MeV (million electron volts or mega electron volts), andrelatively short decay time of 88 nanoseconds (ns)/91% and 258 ns/9%.

GAGG can be grown in the form of large crystal boules of up to 3 inches(about 7.5 centimeters) in diameter from oxides such as cerium dioxide(CeO₂), gadolinium oxide (Gd₂O₃), gallium oxide (Ga₂O₃) and alumina(Al₂O₃) that have a purity of 99.99% or greater using Czochralskimethod. A boule is a single crystal ingot produced by synthetic means.

One of the drawbacks associated with the Czochralski method is that thehigh temperatures (exceeding 1300° C.) used in the production of thecrystal boule result in the decomposition of the gallium oxide to Ga₂Ovapor according to the reaction:Ga₂O₃

Ga₂O+O₂  (1)This is an equilibrium reaction and the presence of additional oxygen inthe reaction chamber reduces the rate of decomposition of the galliumoxide. In other words, the presence of an increased amount of oxygen inthe reaction chamber drives the reverse reaction towards the formationof the gallium oxide rather than towards the formation of the Ga₂Ovapor.

The production of the crystal boule is generally conducted in iridiumcrucibles that are very expensive because of the cost of iridium metal.The use of large amounts of oxygen in the reaction chamber causes theconversion of iridium metal to iridium oxide (which evaporates), whichis undesirable because of the high cost associated with the loss ofiridium metal.

In order to effect a compromise and to obtain the GAGG crystal boulewithout any loss or iridium metal, an off-stoichiometric mix of rawoxide materials having 3 weight percent (wt %) excess of the galliumoxide is used to perform the growth process in an atmosphere containingan excess of 2 volume percent oxygen. The presence of excess galliumoxide compensates for the loss due to evaporation of gallium at theelevated process temperatures.

However, this method has its drawbacks as well. Since the evaporationlosses of gallium are practically difficult to control, the quality ofGAGG crystals may vary significantly. Loss of stoichiometry may causesubstantial non-uniformities in the scintillation characteristics of theGAGG crystals such as: variation in light output, uncontrollablescintillation decay time, and a high level of afterglow, all of whichare undesirable.

There therefore remains a need for a method to produce crystal boules ofthe correct stoichiometry while at the same time reducing the losses ofgallium oxide or of iridium metal.

SUMMARY

Disclosed herein is a method comprising manufacturing a powder having acomposition of formula (1),M¹ _(a)M² _(b)M³ _(c)M⁴ _(d)O₁₂  (1)where O represents oxygen, M¹, M², M³, and M⁴ represents a first,second, third and fourth metal that are different from each other, wherethe sum of a+b+c+d is about 8, where “a” has a value of about 2 to about3.5, “b” has a value of 0 to about 5, “c” has a value of 0 to about 5“d” has a value of 0 to about 1, where “about” is defined as ±10%deviation from the desirable value, where “b” and “c”, “b” and “d” or“c” and “d” cannot both be equal to zero simultaneously, where M¹ is arare earth element including but not being limited to gadolinium,yttrium, lutetium, scandium, or a combination of thereof, M² is aluminumor boron, M³ is gallium and M⁴ is a dopant and comprises one ofthallium, copper, silver, lead, bismuth, indium, tin, antimony,tantalum, tungsten, strontium, barium, boron, magnesium, calcium,cerium, yttrium, scandium, lanthanum, lutetium, praseodymium, terbium,ytterbium, samarium, europium, holmium, dysprosium, erbium, thulium orneodymium, or any combination of thereof; and heating the powder to atemperature of 800 to 1700° C. in an oxygen containing atmosphere tomanufacture a crystalline scintillator.

Disclosed herein too is an article manufactured by a method comprisingmanufacturing a powder having a composition of formula (1),M¹ _(a)M² _(b)M³ _(c)M⁴ _(d)O₁₂  (1)where O represents oxygen, M¹, M², M³, and M⁴ represents a first,second, third and fourth metal that are different from each other, wherethe sum of a+b+c+d is about 8, where “a” has a value of about 2 to about3.5, “b” has a value of 0 to about 5, “c” has a value of 0 to about 5“d” has a value of 0 to about 1, where “about” is defined as ±10%deviation from the desirable value, where “b” and “c”, “b” and “d” or“c” and “d” cannot both be equal to zero simultaneously, where M¹ is arare earth element including but not being limited to gadolinium,yttrium, lutetium, scandium, or a combination of thereof, M² is aluminumor boron, M³ is gallium and M⁴ is a dopant and comprises one ofthallium, copper, silver, lead, bismuth, indium, tin, antimony,tantalum, tungsten, strontium, barium, boron, magnesium, calcium,cerium, yttrium, scandium, lanthanum, lutetium, praseodymium, terbium,ytterbium, samarium, europium, holmium, dysprosium, erbium, thulium orneodymium, or any combination of thereof; and heating the powder to atemperature of 800 to 1700° C. in an oxygen containing atmosphere tomanufacture a crystalline scintillator.

DETAILED DESCRIPTION

Disclosed herein is a method for manufacturing polycrystalline or singlecrystal garnet that comprises gadolinium and gallium (hereinafter“garnet”) and that use gallium oxide as a starting raw material. Thecomposition contains one or more elements in addition to gadolinium andgallium in the garnet. In other words, in its simplest form the garnetcomprises 3 or more elements.

In an embodiment, the garnets have the formula:M¹ _(a)M² _(b)M³ _(c)M⁴ _(d)O₁₂  (1)where O represents oxygen, M¹, M², M³, and M⁴ represents a first,second, third, and fourth metal that are different from each other,where the sum of a+b+c+d is about 8, where “about” is defined as ±10%deviation from the desirable value, where “a” has a value of about 2 toabout 3.5, preferably about 2.4 to about 3.2, and more preferably about3.0, “b” has a value of 0 to about 5, preferably about 2 to about 3, andmore preferably about 2.1 to about 2.5, where “b” and “c”, “b” and “d”,or “c” and “d” cannot both be equal to zero simultaneously, where “c”has a value of 0 to about 5, preferably about 1 to about 4, preferablyabout 2 to about 3, and more preferably about 2.1 to about 2.5, “d” hasa value of 0 to about 1, preferably about 0.001 to about 0.5, and morepreferably 0.003 to 0.3.

In the formula (1), M¹ is a rare earth element including but not beinglimited to gadolinium, yttrium, lutetium, scandium, or a combination ofthereof. M¹ is preferably gadolinium. In an embodiment M² is aluminum orboron, M³ is gallium, and M⁴ is a dopant and comprises one or more ofthallium, copper, silver, lead, bismuth, indium, tin, antimony,tantalum, tungsten, strontium, barium, boron, magnesium, calcium,cerium, yttrium, scandium, lanthanum, lutetium, praseodymium, terbium,ytterbium, samarium, europium, holmium, dysprosium, erbium, thulium orneodymium.

For M¹, some of the gadolinium can be substituted with one or more ofyttrium, lutetium, lanthanum, terbium, praseodymium, neodymium, cerium,samarium, europium, dysprosium, holmium, erbium, ytterbium, orcombinations thereof. In an embodiment, some gadolinium can besubstituted with yttrium. M³ is preferably aluminum.

In an embodiment, the dopant M⁴ includes TI+, Cu+, Ag+, Au+, Pb2+, Bi3+,In+, Sn2+, Sb3+, Ce3+, Pr3+, Eu2+, Yb2+, Nb5+, Ta5+, W6+, Sr2+, B3+,Ba2+, Mg2+, Ca2+, or combinations thereof.

The method comprises manufacturing nanometer and micrometer sizedpowders of the garnet (and associated oxides and hydroxides that can beconverted to the garnet upon heating) and heating these powders to atemperature that is lower than the 1850° C. used for producing singlecrystals in the Czochralski process. In some embodiments, the nanometerand micrometer sized powders are heated to temperatures of 500 to 1700°C. in an oxygen containing atmosphere to form polycrystalline or singlecrystal garnets that have the desired stoichiometry without any loss ofgallium oxide due to evaporation. The powders can optionally be heatedto a temperature of up to 2000° C. to melt them prior to heating them toa temperature of up to 1700° C. in an oxygen containing atmosphere toform polycrystalline or single crystal garnets. The garnet can comprisea combination of a polycrystalline and single crystal material.

Without being limited to theory, it is believed that the gallium ions ineach of the compositions of the formula (1) are strongly bonded to theother elements of the composition and so the energy needed to decomposesuch a molecule is much higher than the energy to decompose the galliumoxide.

The method for manufacturing the nanometer and micrometer-sized powdersof the gadolinium-gallium garnet comprises dissolving the desired metaloxides in the desired stoichiometric ratio in a strong acid. To thesolution comprising the acid and the dissolved metal oxides is added anexcess amount of a strong base. The addition of the base promotes theformation of a precipitate. The precipitate is then separated from thesolution using a separation process to produce the garnet and associatedunreacted oxides in a powdered form. A crystal boule can then bemanufactured from the powders by heating them to temperatures of 500 toless than 2000° C., preferably 850 to 1900° C., and more preferably 900to 1800° C. to melt the powder. Following the melting, the meltedmaterial can be heated to a temperature of 800 to 1700° C., preferably900 to 110° C., and more preferably 950 to 1050° C. in an oxygencontaining atmosphere, to produce in the next stage polycrystalline orsingle crystals that can be used as scintillators.

The raw materials used for the manufacture of gamets generally comprisegallium oxide (Ga₂O₃) and gadolinium oxide (Gd₂O₃), and these materialsare added to a reaction vessel in the desired stoichiometric quantities.The gallium oxide (Ga₂O₃) and gadolinium oxide (Gd₂O₃) are generallyadded to the reaction vessel in a molar ratio of 1:0.5 to 0.5:1,preferably 1:0.75 to 0.75:1, and most preferably 0.9:1 to 1:0.9. In anexemplary embodiment, the gallium oxide (Ga₂O₃) and gadolinium oxide(Gd₂O₃) are generally added to the reaction vessel in a molar ratio of1:1. A preferred form of gallium oxide is β-gallium (III) oxide. Apreferred form of aluminum oxide is α-alumina (α-Al₂O₃).

Additional “metal oxides” such as oxides of cerium, aluminum, scandium,yttrium, lanthanum, lutetium, praseodymium, terbium, ytterbium,neodymium, or combinations thereof can also be added to the reactionvessel in the desired stoichiometric quantities. Exemplary additional“metal oxides” are cerium dioxide (CeO₂), aluminum oxide (Al₂O₃),yttrium oxide (Y₂O₃), lutetium (III) oxide (Lu₂O₃), scandium (III) oxide(Sc₂O₃), or a combination thereof. It is desirable for the metal oxidesused in the manufacturing of the garnet to have a purity of 99.99% orgreater.

In some embodiments, one or more of cerium dioxide (CeO₂), yttrium oxide(Y₂O₃), lutetium (III) oxide (Lu₂O₃), and scandium (III) oxide (Sc₂O₃)can be present in the garnet in addition to the aluminum oxide (A₂O₃).The other metal oxides can be present in a mole ratio of 0.1:1 to 1:0.1,preferably 0.2:1 to 1:0.2, and more preferably 0.5:1 to 1:0.5 withrespect to the gallium oxide (Ga₂O₃).

In an exemplary embodiment, the garnet includes only aluminum oxide asthe additional “metal oxide” component. In these embodiments, the garnetincludes aluminum oxide in a molar ratio of 2:3 to 3:2 with respect tothe number of moles of the gallium oxide (Ga₂O₃). In other embodiments,the garnet can also contain cerium oxide in a molar ratio of 1:3 to 3:1with respect to the number of moles of the gallium oxide (Ga₂O₃), whenthe garnet also contains aluminum oxide.

The raw materials (e.g., a mixture comprising gallium oxide, gadoliniumoxide, aluminum oxide and/or cerium oxide) are then dissolved in astrong acid to form a solution. Examples of strong acids arehydrochloric acid, nitric acid, sulfuric acid, or a combination thereof.In an exemplary embodiment, the strong acid is hydrochloric acid presentin an amount of 25 to 50 mole percent, preferably 30 to 40 mole percentin water.

The solution is prepared by agitation the raw materials in thehydrochloric acid. Agitation can be accomplished by stirring, use ofultrasonic sonication, sparging, physical vibration, or combinationsthereof. The solution can be manufactured at any temperature though roomtemperature is preferred.

After the oxides have dissolved, dopants can be added to the solution.Suitable dopants are cerium, aluminum, scandium, yttrium, lanthanum,lutetium, praseodymium, terbium, ytterbium, neodymium, or a combinationthereof. These dopants can be added to the solution in the form of therespective metal halides. Preferred halides are chlorides, bromides, ora combination thereof. It is to be noted that these dopants can be addedto the solution even if it contains a certain amount of the dopantalready previously added in the form of a metal oxide.

For example, cerium can be added to the solution in the form of ceriumchloride, cerium bromide, or a combination thereof, even if the solutioncontains cerium that was previously added in the form of cerium oxide asdetailed above.

The metal halide can be added as a dopant to the solution of the garnetin a mole ratio of 0.1:1 to 1:0.1, preferably 0.2:1 to 1:0.2, and morepreferably 0.5:1 to 1:0.5 with respect to the number of moles of galliumoxide (Ga₂O₃).

The solution is then treated with an excess of a strong base in areaction vessel to facilitate a precipitation of the dissolved metaloxides. The solution is added to the base in a reaction vessel understrong agitation. Examples of strong bases are ammonium hydroxide,ammonium bicarbonate, potassium hydroxide, sodium hydroxide, or thelike, or a combination thereof. The strong base is dissolved in water inan amount of 15 to 50 mole percent, preferably 20 to 40 mole percent.

The molar ratio of acid present in the solution to the base is greaterthan 1:1.10, preferably greater than 1:1.20, and more preferably greaterthan 1:1.50.

The addition of the solution to the base causes a precipitation of thegarnet. The precipitate is subjected to a separation process to extractthe garnet from the remainder to the solution. Separation processesinclude centrifugation, filtration, decantation, or a combinationthereof. Filtration is preferred.

The filtrate is subjected to additional washing steps with water toremove traces of acid, salts, and base from the precipitate. Theprecipitate in powdered form comprises the garnet (along with one ormore additional elements) as well as oxides and hydroxides of theoriginal metals used in the reaction vessel. The precipitate obtainedafter the separation of the garnet is in the form of particles that havea particle size in the nanometer range and in the micrometer range. Theparticles have an average particle size that range from 5 nanometers to500 micrometers, preferably 10 nanometers to 50 micrometers, and morepreferably 1 to 20 micrometers. The radius of gyration of the particlesis measured to determine average particle size. Light scattering orelectron microscopy can be used to determine the particle size.

The powders can be optionally further pulverized in a ball mill, rollmill or other pulverizing device. The pulverized powders can then besubjected to an optional sieving process if it is desirable to useparticles of a particular size.

The powders of the garnet are then processed at temperatures of 800 to1700° C., preferably 900 to 1100° C., and more preferably 950 to 1050°C. in an oxygen containing atmosphere, to produce in the next stagepolycrystalline or single crystals that can be used as scintillators.

Single crystals can be produced by the Czochralski method, the Bridgmantechnique, the Kyropoulos technique, and the Verneuil technique.

In the Czochralski method, the powder to be grown is melted under acontrolled atmosphere in a suitable non-reacting container. Bycontrolling the furnace temperature up to 1700 C°, the material ismelted. A seed crystal is lowered to touch the molten charge. When thetemperature of the seed is maintained very low compared to thetemperature of the melt (by a suitable water cooling arrangement), themolten charge in contact with the seed will solidify on the seed. Thenthe seed is pulled at a controlled rate. The majority of crystals areproduced by pulling from the melt. Crystals of dimensions 3 to 40centimeters can be grown using this method.

In the Bridgman (pulling method) technique, the material is melted in avertical cylindrical container (called an ampoule), tapered conicallywith a point bottom. The container is lowered slowly from the hot zoneof the furnace having a temperature up to 1700° C. into the cold zone.The rates of movement for such processes range from about 1-30 mm/hr.Crystallization begins at the tip and continues usually by growth fromthe first formed nucleus. Due to a directed and controlled coolingprocess of the cast, zones of aligned crystal lattices are created. Inother words, a single crystal can be created.

In the Kyropoulos technique, the crystal is grown in a larger diameterthan in the aforementioned two methods. As in the Czochralski method,here too a seed is brought into contact with the melt and is not raisedmuch during the growth, i.e., part of the seed is allowed to melt and ashort, narrow neck is grown. After this, the vertical motion of the seedis stopped and growth proceeds by decreasing the power into the melt.

In the Verneuil technique, (flame fusion) a fine dry powder of size 1 to20 micrometers of the material to be grown is shaken through the wiremesh and allowed to fall through the oxy-hydrogen flame. The powdermelts and a film of liquid is formed on the top of a seed crystal. Thisfreezes progressively as the seed crystal is slowly lowered. The art ofthe method is to balance the rate of charge feed and the rate oflowering of the seed to maintain a constant growth rate and diameter. Bythis method ruby crystals are grown up to 90 millimeters in diameter.This technique is widely used for the growth of synthetic gems andvariety of high melting oxides.

Examples of polycrystalline or single crystals grown by this method havethe following formulas—Gd₃Al₂Ga₃O₁₂ (GAGG—gadolinium-aluminum-galliumgarnet), Gd₃Ga_(2.5)Al_(2.5)O₁₂ (GGAG—gadolinium-gallium-aluminumgarnet), Gd_(1.5)Y_(1.5)Ga_(2.5)Al_(2.5)O₁₂(GYGAG—gadolinium-yttrium-gallium-aluminum garnet), Gd₃Sc₂Ga₃O₁₂(GSGG—gadolinium-scandium-gallium-garnet), orGd_(1.5)Lu_(1.5)Al_(1.5)Ga_(1.5)O₁₂. Each of the polycrystalline orsingle crystals represented by the aforementioned formulas can be dopedwith cerium if desired.

The polycrystalline or single crystals manufactured using nanometers andmicrometer size powders have a more consistent stoichiometry whencompared with materials manufactured using melted oxide compounds.Scintillator materials manufactured by this method can be used inimaging devices such as, for example, positron emission tomography,computed tomography or single photon emission computed tomographymachines.

It is to be noted that all ranges detailed herein include the endpoints.Numerical values from different ranges are combinable. The compositions,methods, and articles can alternatively comprise, consist of, or consistessentially of, any appropriate components or steps herein disclosed.The compositions, methods, and articles can additionally, oralternatively, be formulated so as to be devoid, or substantially free,of any steps, components, materials, ingredients, adjuvants, or speciesthat are otherwise not necessary to the achievement of the functionand/or objectives of the compositions, methods, and articles.“Combinations” is inclusive of blends, mixtures, alloys, reactionproducts, and the like. The terms “a” and “an” and “the” do not denote alimitation of quantity, and are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. “Or” means “and/or” unless clearly statedotherwise. Reference throughout the specification to “some embodiments”,“an embodiment”, and so forth, means that a particular element describedin connection with the embodiment is included in at least one embodimentdescribed herein, and may or may not be present in other embodiments. Inaddition, it is to be understood that the described elements may becombined in any suitable manner in the various embodiments.

While the invention has been described with reference to someembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing fromessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method comprising: dissolving gallium oxide,gadolinium oxide, aluminum oxide or boron oxide, and optionally at leastone oxide of cerium, scandium, yttrium, lanthanum, lutetium,praseodymium, terbium, ytterbium, and neodymium in an acid to form asolution; where the acid is hydrochloric acid, nitric acid, sulfuricacid or a combination thereof; adding a dopant to the solution, wherethe dopant is a halide of cerium, aluminum, scandium, yttrium,lanthanum, lutetium, praseodymium, terbium, ytterbium, neodymium, or acombinations thereof; manufacturing a powder having a composition offormula (1):M¹ _(a)M² _(b)M³ _(c)M⁴ _(d)O₁₂  (1) where O represents oxygen, M¹, M²,M³, and M⁴ represent a first, second, third, and fourth metal that aredifferent from each other, the sum of a+b+c+d is about 8, where “a” hasa value of about 2 to about 3.5, “b” has a value of 2 to about 5 “c” hasa value of 1 to about 5, and “d” has a value of 0.001 to about 1, M¹ isa rare earth element of gadolinium and optionally at least one ofcerium, scandium, yttrium, lanthanum, lutetium, praseodymium, terbium,ytterbium, and neodymium, M² is aluminum or boron M³ is gallium, and M⁴is a dopant and is one of cerium, aluminum, yttrium, scandium,lanthanum, lutetium, praseodymium, terbium, ytterbium, neodymium, andcombinations thereof; heating the powder to a temperature of 500 to2000° C. to melt the powder; and heating the powder to a temperature of800-1700° C. in an oxygen containing atmosphere to manufacture acrystalline scintillator; wherein the melting of the powder is conductedprior to the heating of the powder to a temperature of 800-1700° C. inan oxygen containing atmosphere to manufacture a crystallinescintillator.
 2. The method of claim 1, where M¹ is gadolinium and M² isaluminum.
 3. The method of claim 1, where “a” has a value about 2.4 toabout 3.2, “b” has a value of about 2 to about 3, “c” has a value ofabout 1 to about 4, and “d” has a value of about 0.001 to about 0.5. 4.The method of claim 3, where “a” has a value of about 3, “b” has a valueof about 2.1 to about 2.5, “c” has a value of about 2 to about 3, and“d” has a value of about 0.003 to about 0.3.
 5. The method of claim 1,where the acid is hydrochloric acid; where the hydrochloric acid ispresent in an amount of 25 to 50 mole percent in water.
 6. The method ofclaim 1, further comprising adding the solution to an excess of a strongbase and precipitating the garnet from the solution to form a powder. 7.The method of claim 6, where the base is ammonium hydroxide, ammoniumbicarbonate, potassium hydroxide, sodium hydroxide or a combinationthereof.
 8. The method of claim 6, further comprising washing the powderand subjecting it to further grinding.
 9. The method of claim 8, wherethe powder has average particle sizes of 5 nanometers to 500micrometers.
 10. The method of claim 1, where the crystallinescintillator comprises a single crystal, a polycrystalline material or acombination thereof.
 11. The method of claim 10, where the singlecrystal is produced by the Czochralski method, the Bridgman technique,the Kyropoulos technique, or the Verneuil technique.